As wafer sizes increase and geometries shrink, specifications for electronic devices become more stringent. The reason for these more stringent specifications is to obtain better yields and fewer losses after many value added steps have been completed in fabricating the electronic devices. One such important specification for the future is surface texture.
Although surface texture has received little attention in the microelectronics industry until recently, texture or micro-roughness can influence gate oxide integrity, as well as the perfection of wafer bonding. In some cases, surface texture has been known to influence the effectiveness of wafer cleaning. Surface texture is usually reported as root-mean-square (RMS) roughness in a direction that is perpendicular to the surface of the wafer. At present, there is no precise measurement technique to determine surface texture on regular production wafers in a manufacturing environment.
Currently several lab techniques attempt to measure the texture on wafers. Some of these techniques are known by the names of light scattering topography (LST), stylus profilometry, phase shift interferometry, and atomic force microscopy. These methods, as well as all other current methods, are point location measurement methods and do not scan the entire wafer surface. Moreover, the known methods are time consuming and can only measure minute parts of the wafers. For example, a measuring tool known as the WYCO samples one square millimeter of the wafer surface in approximately two minutes. Hence, it takes many hours to sample a sufficient amount of surface area to yield a meaningful surface texture measurement for a single wafer.
With increasing wafer diameter, the surface area increases as a square function. For example, a 200 mm wafer has a surface area of 31,428 sq mm. Known procedures use the millimeter sampling approach to sample usually nine arbitrarily selected one millimeter square spaces. On a wafer whose surface area is 31,428 square millimeters, however, a sample measurement of nine separate one millimeter square areas is quite meaningless.
Consequently, there is no existing method to perform meaningful measurements or inspections of surface microroughness or surface texture.
Still a further limitation that exists in conventional devices relate to the mechanical aspects of measuring highly magnified images. Because the known approaches measure such small areas on the surface, mechanical vibrations severely distort the measurement accuracies.
A third problem of known methods and systems are the effects that any contamination on the surface have on the sample measurement results. Measurements of the highly magnified images that known methods use may include fine particle contamination. In these measurements, the contamination can greatly distort the accuracy of the measurements that these approaches yield.
Yet another problem of conventional surface texture measurement techniques is that they are often destructive to the measured wafer. In conventional methods, samples will be either contaminated or the surface may be damaged in the measurement process. The wafers that are sampled in conventional techniques generally must be removed from the production processes.